The Trump administration will provide heavier weapons to Syria’s Kurds as they and their allies move closer to an attack on the key Islamic State stronghold of Raqqa, U.S. officials said Tuesday.

The decision is meant to accelerate the Raqqa operation, but it clashes with the Turkish government’s view that the Syrian Kurdish group known as the YPG is an extension of Kurdish terrorist organization that operates in Turkey.

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The U.S. sees the Kurds as its most effective battlefield partner against IS in northern and eastern Syria.

After lengthy deliberations, the administration approved plans to provide additional weaponry to the Kurds. A full list wasn’t immediately available, but officials had indicated in recent days that 120mm mortars, machines guns, ammunition and light armored vehicles were possibilities. They said the U.S. would not provide artillery or surface-to-air missiles.

The U.S. officials who disclosed the Trump administration decision weren’t authorized to publicly discuss the matter and demanded anonymity. They described no firm timeline, with the American intention to provide the new weapons to the Syrian Kurds as soon as possible.

A congressional aide said officials informed relevant members of Congress of the decision on Monday evening.

Senior U.S. officials including Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, have met repeatedly with Turkish officials to try to work out an arrangement for the Raqqa assault that would be acceptable to Ankara. The Turks have insisted that the Syrian Kurds be excluded from that operation, but U.S. officials insisted there was no real alternative.

Turkish President Recep Tayyip Erdogan is to visit President Donald Trump in Washington next week. An Erdogan adviser, Ibrahim Kalin, met on Tuesday with Thomas Shannon, the State Department No. 2 official.

And in Denmark earlier Tuesday, Defense Secretary Jim Mattis said he had useful discussions with Turkey and described the two countries as working out differences over a U.S. alliance with Syrian Kurds in fighting Islamic State militants.

“That’s not to say we all walk into the room with exactly the same appreciation of the problem or the path forward,” Mattis told reporters after meeting with officials from more than a dozen nations also fighting IS. Basat Ozturk, a senior Turkish defense official, participated.

“We’re going to sort it out,” Mattis said. “We’ll figure out how we’re going to do it.”

Tensions escalated last month when Turkey conducted airstrikes on Kurdish bases in Syria and Iraq. The Turkish military said it killed at least 90 militants and wounded scores. The Kurdish group in Syria said 20 of its fighters and media activists were killed in the strike, which was followed by cross-border clashes.

The instability has concerned Washington, which fears it will slow the effort to retake Raqqa.

“We’ve been conducting military and diplomatic dialogue with the Turks and it was a very, very useful discussion today,” Mattis said at a press conference with Danish Defense Minister Claus Hjort Frederiksen.